1. Field of the Invention
The present invention relates to a dielectric element for positioning on an examination subject for locally influencing the B1 field distribution during magnetic resonance recording. The invention additionally relates to a method for acquiring magnetic resonance data from an examination subject, wherein such a dielectric element is positioned on the examination subject for locally influencing the B1 field distribution by homogenizing the B1 field of a magnetic resonance apparatus.
2. Description of the Prior Art
Magnetic resonance tomography is a technique that is widely used for obtaining images inside the body of an examination subject. In order to obtain an image with this modality, i.e. to produce a magnetic resonance recording of an examination subject, the patient's body or a body part thereof under examination must first be exposed to a highly homogeneous static basic magnetic field (usually termed the B0 field), which is generated by a basic field magnet of the magnetic resonance measuring device. During recording (data acquisition) of the magnetic resonance images, rapidly switched gradient fields generated by gradient coils are superimposed on this basic magnetic field for spatial coding. In addition, an RF antenna is used to radiate RE pulses of a defined field strength into the examination volume in which the examination subject is located. The magnetic flux density of these RF pulses is usually referred to as B1. The pulse-shaped RF field is generally therefore also known as the B1 field. By means of these RF pulses, the nuclear spins of the atoms in the examination subject are excited in such a way that they are deflected from their equilibrium position that is parallel to the basic magnetic field B0 by a so-called “excitation flip angle” (hereinafter referred to as “flip angle” for short). The nuclear spins then precess around the direction of the base magnetic field B0. The magnetic resonance signals generated thereby are detected by radio-frequency reception antennas. The reception antennas can be either the same antennas as were used to emit the radio-frequency pulses or separate receiving antennas. The magnetic resonance images of the examination subject are generated on the basis of the received magnetic resonance signals. Each image point in the magnetic resonance image is assigned to a small body volume known as a “voxel” and each brightness or intensity value of the images points is linked to the signal amplitude of the magnetic resonance signal received from this voxel. The relationship between a resonantly radiated RF pulse with field strength B1 and the flip angle attained is given by the equation
                    α        =                              ∫                          t              =              0                        τ                    ⁢                      γ            ·                                          B                1                            ⁡                              (                t                )                                      ·                                                  ⁢                          ⅆ              t                                                  I      wherein γ is the gyromagnetic ratio, which can be considered to be a fixed material constant for most nuclear spin examinations, and τ is the effective duration of the radio-frequency pulse. The flip angle attained by the RF pulse and therefore the strength of the magnetic resonance signal consequently depend not only on the duration of the RF pulse but also on the strength of the radiated B1 field. Spatial fluctuations in the field strength of the excited B1 field therefore result in unwanted variations in the received magnetic resonance signal, which may distort the measurement result.
Disadvantageously, at high magnetic field strengths—which are necessarily present because of the required base magnetic field B0 in a magnetic resonance apparatus—the RF pulses exhibit an inhomogeneous penetration behavior into conductive and dielectric media such as tissue. This causes the B1 field to vary markedly within the measuring volume. Particularly in the case of ultra-high field magnetic resonance examinations in which more modern magnetic resonance systems with a basic magnetic field of three teslas or more are used, special measures therefore must be taken to achieve, throughout the volume, a maximally homogeneous distribution of the transmitted RF field of the radio-frequency antenna.
A simple but effective approach to solving the problem is to suitably modifying the (di-)electric environment of the examination subject in order to eliminate unwanted inhomogeneities. For this purpose, for example, dielectric elements of defined dielectric constant and conductivity can be positioned in the examination volume e.g. immediately at or on the patient. The material of these dielectric elements must have a high dielectric constant, preferably ε≧50, the dielectric material thereby ensuring dielectric focusing. The material of the dielectric element, however, must not be overly conductive, as due to the skin effect an excessively high conductivity results in high eddy currents particularly in the surface region of the dielectric element, thereby producing a shielding effect which in turn attenuates the dielectric focusing effect. Using such dielectric elements it is possible, for example, to compensate the RF field minima typically occurring during magnetic resonance examinations of a patient in the chest and abdominal region by placing, on the patient's chest and abdomen, corresponding dielectric elements which in turn compensate the minima by locally increasing the penetrating RF field.
A plastic pouch containing distilled water with a dielectric constant of ε≈80 and a conductivity of approx. 10 μS/cm has conventionally been used as a dielectric element. Using such water-filled “dielectric cushions” has the undesirable side-effect that they are visible in the magnetic resonance recordings. Additionally, due to foldover effects, the dielectric element may not be imaged within the magnetic resonance recording at the position where it is actually positioned in real space. Thus, for example, the foldover effect may cause the cushion to be shown at the lower edge instead of at the upper edge of the MR image. This creates the impression in the magnetic resonance recordings that the dielectric element is not on the patient's body but in the patient's body. It is indeed basically possible using so-called oversampling methods to record an image in such a way that the dielectric element is in the correct position. In such a case the dielectric element can be excised during subsequent processing or an image detail can be selected which does not include the dielectric element at all. Such oversampling methods, however, are extremely time-consuming and therefore prolong the measurement time.
In addition, in the German application DE 10 2004 015 859 A1 (corresponding to U.S. Ser. No. 11/095,159 filed Mar. 31, 2005) it is proposed, as a dielectric element, to employ a “dielectric cushion” filled with a relaxation agent, in particular a gel filling, instead of the above described distilled water filling, enabling the above described problem to be partially solved. However, it has been found that these cushions have an undesirable effect on the B0 field. This can cause local geometric image distortions and, when using spectrally selective RF pulses, an inhomogeneous fat saturation. In addition, the homogenization of the transmitted RF field is not yet optimum.